Function.applyN

addresses #155

Author: matthewleon

src/Data/Function.purs

@@ -8,6 +8,9 @@ module Data.Function
   ) where
 
 import Control.Category (id, compose, (<<<), (>>>))
+import Data.Boolean (otherwise)
+import Data.Ord ((<=))
+import Data.Ring ((-))
 
 -- | Flips the order of the arguments to a function of two arguments.
 -- |
@@ -87,3 +90,13 @@ infixl 1 applyFlipped as #
 -- | ```
 on :: forall a b c. (b -> b -> c) -> (a -> b) -> a -> a -> c
 on f g x y = g x `f` g y
+
+-- | Applies a function to an argument `n` times.
+-- |
+-- | If n is less than or equal to 0, the function is not applied.
+applyN :: forall a. (a -> a) -> Int -> a -> a
+applyN f n = go n
+  where
+  go n' acc
+    | n' <= 0    = acc
+    | otherwise = go (n' - 1) $ f acc

src/Data/Ord.purs

@@ -15,7 +15,6 @@ module Data.Ord
   ) where
 
 import Data.Eq (class Eq, class Eq1)
-import Data.Function (on)
 import Data.Ord.Unsafe (unsafeCompare)
 import Data.Ordering (Ordering(..))
 import Data.Ring (class Ring, zero, one, negate)
@@ -106,6 +105,9 @@ infixl 4 greaterThanOrEq as >=
 -- | Compares two values by mapping them to a type with an `Ord` instance.
 comparing :: forall a b. Ord b => (a -> b) -> (a -> a -> Ordering)
 comparing f = compare `on` f
+  where
+  -- we need to redefine this to avoid a dependency cycle with `Data.Function`
+  on f g x y = g x `f` g y
 
 -- | Take the minimum of two values. If they are considered equal, the first
 -- | argument is chosen.